A laser is often required which has its long term frequency stabilized to a high precision. One way to do this is to lock the frequency to an external source, such as the resonant absorption frequency of a gas through which the laser beam is directed. This is unsatisfactory for two reasons. First, the desired laser frequency may be different from that of the resonant frequency of any conveniently available reference material. Second, dithering the laser so as to induce changes in amplitude in the beam as it emerges from the reference material will itself contaminate the spectral purity of the laser by adding side bands to the frequency spectrum of the laser.